Electronic systems utilize dimmers to modify output power delivered to a load. For example, in a lighting system, dimmers provide an input signal to a lighting system, and the load includes one or more light sources such as one or more light emitting diodes (LEDs) or one or more fluorescent light sources. Dimmers may also be used to modify power delivered to other types of loads, such as one or more motors or one or more portable power sources. The input signal represents a dimming level that causes the lighting system to adjust power delivered to a lamp, and, thus, depending on the dimming level, increase or decrease the brightness of the lamp. Many different types of dimmers exist. In general, dimmers use a digital or analog coded dimming signal that indicates a desired dimming level.
Conventionally, dimmers are constructed with a triode for alternating current (“TRIAC”) device to modulate a phase angle of each cycle of an alternating current (“AC”) supply voltage. The TRIAC is placed in series with the power connection, acting as low impedance series device when in the “on” state, and as an open circuit when in the “off” state. That is, the TRIAC cuts the AC waveform during a certain time. If the cut occurs at the beginning of the cycle, the dimmer is called “leading edge” (LE). If the cut occurs at the end of the cycle, the dimmer is called a “trailing edge” (TE) dimmer.
FIG. 1 is a graph illustrating various waveforms of a conventional leading edge (LE) dimmer system. A Vline voltage line illustrates a live voltage supplied to a circuit after passing through a dimmer and rectifier component. As described above, in a leading edge system the beginning of each cycle of the voltage is cut off. Additionally, FIG. 1 shows various currents within a device, including the dimmer current, Idimmer.
When the load is drawing no current, the rectifier, together with capacitance present in the circuit, maintains a nearly constant voltage at the line output when the line voltage decreases. Digitally-controlled converters, however, require information about the line voltage zero crossing to synchronize their operation to the line frequency. The converter typically draws current only during a portion of the line cycle to feed the load, such as while the dimmer is on. The shape of the rectified line voltage may be recovered if an additional current, such as a “probe” current, is applied such that the internal capacitances are discharged and the rectifier output follows the input voltage. Also, when the dimmer is off, it is necessary to discharge the dimmer timing network to guarantee a repeatable firing angle. This is performed by presenting to the line voltage a low impedance path.
This additional current, and other currents, are drawn from an AC line voltage through a controlled device, such as a current mode digital-to-analog converter (DAC). The power dissipated in this device is proportional to the current and the voltage across it. For example, as shown on FIG. 1, a dimmer current peaks between time A and time B at the start of the leading edge (LE) of the line voltage, Vline. This current, Idimmer, is conventionally dissipated as power within the controlled device. If the DAC is part of an integrated circuit, the power may be excessive unless the voltage is reduced to acceptable levels. One solution is to use an external discrete active device (FET) that provides a voltage drop, but the FET is a costly device and may consume a large amount of component space when built to handle the power ratings required to dissipate the currents.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved integrated circuits, particularly for lighting devices. Embodiments described here address certain shortcomings but not necessarily each and every one described here or known in the art.